Layered, absorbent structure

ABSTRACT

A thin, layered, absorbent structure comprises first and second layers of fibrous material and an intermediate layer comprising particles of a hydrogelling, absorbent material and particles of a thermoplastic, polymeric, organic material, the first and second fibrous layers extend beyond the intermediate layer laterally to form longitudinal edge portions. 
     The two fibrous layers are bound together with the intermediate layer between them by the melting of the particles of thermoplastic, polymeric, organic material and by means of a continuous line of adhesive extending longitudinally on each longitudinal edge portion.

The present invention relates to layered, absorbent structuresconstituted by fibrous layers with particles of hydrogelling absorbentmaterial.

The structures may be used as absorbent elements in disposable absorbentarticles such as sanitary towels, dressings, babies' nappies and thelike.

Disposable absorbent articles are well known and all have absorbentelements for absorbing and retaining body fluids; an absorbent elementmust be able to acquire liquid rapidly and to distribute it internallyso as to prevent leakages and must also have a good capacity to retainthe fluids when subjected to the normal pressures of use.

Absorbent elements made mainly of hydrophilic fibrous material such as,for example, pads of cellulose fibres, layers of wadding, or the likegenerally have satisfactory characteristics as regards theirliquid-absorption rate and can distribute the liquid effectively withinthem but are very ineffective from the point of view of retention whensubjected to the normal pressures of use.

The use of hydrogelling absorbent materials in combination withhydrophilic fibres in order to increase the absorption and retentioncapacities of the absorbent elements is known.

Hydrogelling absorbent materials, commonly known as superabsorbents, arepolymers which can swell up and absorb large quantities of liquid,particularly water, or also, to a lesser extent, body fluids.

They also have the particular property that they retain the fluids evenunder moderate pressure; owing to this characteristic, their use inabsorbent elements for disposable absorbent articles has been proposedfor some time.

The high absorption capacity of superabsorbents is not, however,combined with similarly rapid absorption and this may adversely affectthe performance of absorbent articles incorporating these substances.

In fact, superabsorbents may give rise to a phenomenon defined as "gelblocking"; when a particle of superabsorbent is in contact with theliquid, its external surface starts to absorb the liquid and swells upobstructing the transmission of the liquid into the particle itself; theliquid can penetrate further into the still-dry core of the particleonly by means of a very slow diffusion mechanism.

This phenomenon may prevent full use being made of the large absorptioncapacities of superabsorbent substances.

Leaving out of consideration the type of superabsorbent material and theshapes and sizes of the particles, "gel blocking" is generallyencouraged by the formation of agglomerations of superabsorbentparticles within the absorbent structure, which may occur either beforeuse, when the particles are still dry, or during use, when the particlesstart to absorb the liquid and swell up.

In any case, with the use of hydrogelling absorbent materials, it ispossible to produce absorbent elements which contain less hydrophilicfibres for a given absorption capacity and which consequently havesmaller dimensions, particularly widths, than conventional absorbentelements made of fibres alone.

Structures have been formed in which the fibres and the particles ofhydrogelling, absorbent material are disposed in separate, generallyvery thin, superposed layers.

Many particular forms of layered, absorbent structures of this type, inwhich the fibrous material is represented by one of more layers ofwadding, absorbent paper or non-woven fabric, and in which the particlesof hydrogelling absorbent material are incorporated in the structure invarious ways, are known in the art.

In layered structures which are formed dry, the at least two fibrouslayers are bound together solely by the intertwining of fibres and theparticles of hydrogelling absorbent material between them are held inposition by the two fibrous layers joined together; a better connectionbetween the fibrous layers can be achieved by subjecting the structureto an embossing process which encourages the fibres to intertwine bycausing slight relative slippage between the layers; a solution of thistype is described, for example, in U.S. Pat. No. 4,578,068.

Alternatively, the layered structure may be formed damp by theapplication of water, or preferably steam, and pressure; the connectionbetween the fibrous layers is achieved mainly by virtue of the particlesof hydrogelling, absorbent material which become sticky and act as anadhesive between the fibrous layers.

In neither case is the structure very stable and the fibrous layers mayseparate, for example, along their edges, leaving the particles ofhydrogelling absorbent material free to slide between the surfaces ofthe layers between which they are included; the particles ofhydrogelling absorbent material may thus become concentrated locallythereby encouraging, and possibly aggravating, the establishment of the"gel blocking" phenomenon in use, and may also escape from the edges ofthe structure, in any case reducing the absorption capacity of thestructure.

Another solution for the formation of a layered absorbent structureprovides for the use of an adhesive, for example, of the hot meltingtype, applied to the surface of one of the fibrous layers with the dualpurpose of bonding the two fibrous layers together and simultaneouslyfixing the particles of superabsorbent material between them.

The use of an adhesive may, however, affect the absorptioncharacteristics both of the fibrous layer to which the adhesive isapplied, and of the superabsorbent particles which come into contactwith the adhesive.

In general, therefore, it is necessary not to use an excessive quantityof adhesive and consequently not to eliminate completely the possibilityof the loss of superabsorbent material from the edges of the layeredstructure.

For this reason, layered absorbent structures of the type described areformed directly on the production line for the absorbent articles inwhich they are to be incorporated, since it is not convenient to producethem independently as semi-finished products.

The problem of the loss of hydrogelling absorbent material along theedges is common to all the layered structures described up to now andhas been solved in various ways in known products; for example, bysurrounding the structure completely with a layer of wadding, whichinvolves the addition of a further element, or by the use of a singlelayer of fibrous material on which the adhesive and the superabsorbentmaterial are distributed only on a central longitudinal strip andsubsequently folding the two side portions so that they partiallyoverlap approximately on the longitudinal axis.

This solution solves the problem of the loss of hydrogelling absorbentmaterial from the edges of the structure without requiring the presenceof additional elements but does not allow different materials to be usedfor the two fibrous layers. Moreover, it involves the use of a largerquantity of material than would be needed simply to superpose two layersof the same width as the finished product.

There remains therefore the problem of providing a thin, layered,absorbent structure which does not have the disadvantages describedabove connected with the method of joining together the various layersof which it is made up and, preferably, of reducing the quantity ofmaterial so as to produce a thinner and lighter structure.

The object of the present invention is to improve the absorption andstability characteristics of thin, layered, absorbent structurescontaining hydrogelling, absorbent material for use in disposableabsorbent articles.

According to the present invention, this object is achieved by virtue ofa thin, layered, absorbent structure having the specific characteristicsrecited in the following claims.

Further subjects of the invention are an article comprising thestructure as well as a method for the manufacture thereof.

In summary, the invention relates to a thin, layered, absorbentstructure formed by at least two fibrous layers comprising between thema layer of particles of hydrogelling, absorbent material, the twofibrous layers being joined together by particles of thermoplastic,polymeric, organic material distributed and mixed with the hydrogelling,absorbent material, and by two lines of adhesive disposed along thelongitudinal edges of the structure.

Further characteristics and advantages of the invention will becomeclear from the description which follows, given purely by way ofnon-limiting example, with reference to the appended drawings, in which:

FIG. 1 is a perspective view of a thin, layered, absorbent structureformed according to the present invention, with one of the two layerspartially raised,

FIG. 2 is a sectional view of the thin, layered, absorbent structure,taken on the line II--II of FIG. 1,

FIG. 3 is a perspective view of a continuous strip of the thin, layered,absorbent structure of the present invention rolled in the form of aroll,

FIG. 4 is a plan view of a continuous strip of an alternativeconfiguration of the thin, layered, absorbent structure,

FIG. 5 is a schematic view of a device for producing a thin, layered,absorbent structure according to the present invention, and

FIG. 6 is a plan view of a disposable sanitary towel using a thin,layered, absorbent structure according to the present invention as anabsorbent element.

The thin, layered, absorbent structures of the present invention will bedescribed herein in relation to their use in disposable absorbentarticles; the articles are worn by the user in direct contact with thebody; their purpose is to absorb body fluids and they are then thrownaway after a single use.

FIGS. 1 and 2 show a preferred configuration of a thin, layered,absorbent structure formed according to the present invention. In FIG.2, one of the layers is partially raised to show its construction moreclearly.

In FIG. 1 it is possible to distinguish a first fibrous layer 1 and asecond fibrous layer 2 in the form of two continuous strips of the samewidth, which are superposed so that their respective longitudinal edges3 and 4 coincide; the fibrous layers may be made of various materialssuch as, for example, paper, wadding, or non-woven fabric; they arepreferably made of dry-formed layers, generally referred to as "airlaid" layers in English, of short cellulose fibres having a basic weightof between 20 g/m² and 150 g/m².

Between the two fibrous layers 1 and 2 there is an intermediate layer 5made of a mixture of particles of hydrogelling, absorbent material 6 andparticles of a thermoplastic, polymeric, organic material 7; the widthof the intermediate layer 5 is less than the widths of the two outerfibrous layers 1 and 2 which extend beyond the intermediate layer 5laterally forming two longitudinal edge portions 8 at their respectivelongitudinal edges 3 and 4.

The two outer fibrous layers 1 and 2 are bonded together in the centralregion in which the intermediate layer 5 is present by the applicationof heat and moderate pressure to melt the particles 7 of thermoplastic,polymeric, organic material present in the intermediate layer 5, mixedwith the particles 6 of hydrogelling, absorbent material. As can best beseen in FIG. 2, which is a section of the absorbent structure taken onthe line AA' of FIG. 1, the bond between the fibrous layers 1 and 2 isformed at discrete spaced-apart points 9 generated by the melting of theindividual particles 7 of hot melting, polymeric, organic material; asit melts, the polymeric material bonds the fibres of the two adjacentlayers 1 and 2 together.

In practice, the particles 6 of hydrogelling, absorbent material areaffected only to a very marginal extent by the melting of the particles7 of hot melting, polymeric, organic material, and thus remain trappedbetween the two fibrous layers without being substantially bondedthereto, being only minimally affected by the presence of anynon-absorbent material directly in contact with them.

Moreover, the overall surface areas of the bond points represent a smallfraction of the surface areas of the fibrous layers 1 and 2 theabsorption characteristics of which thus remain almost unchanged.

Two continuous lines 10 of adhesive are also applied to the two sides ofthe intermediate layer 5 on the longitudinal edge regions 8 of the twoouter fibrous layers 1 and 2 so as to eliminate any further possibilityof particles of hydrogelling, absorbent material 6 escaping from thelongitudinal edges of the layered structure, which correspond to thesuperposed edges 3 and 4 of the two fibrous layers, and also reinforcingthe connection between the fibrous layers themselves.

The hydrogelling material, which is preferably distributed in the formof particles 6, may be made of inorganic or organic substances such ascross-linked polymers, all known from the prior art.

The average dimensions of the particles 6, given as a weighted averageof the smallest dimensions of the individual particles, may be between100 microns and 800 microns.

The quantity of the hydrogelling absorbent material 6 distributed toform the intermediate layer 5, together with the thermoplastic polymericorganic material 7, may be between 30 g/m² and 150 g/m².

The finely-divided, thermoplastic, polymeric, organic material has thepurpose of bonding the two fibrous layers 1 and 2 together by meltingand forming discrete, spaced-apart bond points 9 between the fibres ofthe two layers.

For this purpose, the quantity of thermoplastic, polymeric, organicmaterial distributed and mixed with the hydrogelling, absorbent materialis between 5 g/m² and 30 g/m².

The thermoplastic, polymeric, organic material can preferably be meltedat a temperature such as not to interfere with the characteristics ofthe other components of the layered structure, particularly the fibrouslayers and the hydrogelling, absorbent material; moreover, it must havefluidity characteristics such as to enable the bond between the fibresto be formed rapidly; in other words, when it melts, the individualparticle of thermoplastic polymeric organic material is interpenetratedby fibres belonging to both the fibrous layers and when it subsequentlysets, it forms a single bond point between the two layers 1 and 2.

It has been found that these preferred characteristics can be achievedby a thermoplastic, polymeric, organic material 7 having a melt flowindex (M.F.I.), evaluated by the ASTM method D 1238-85 under conditions190/2.16, of at least 25 g/10 min., preferably at least 40 g/10 min, andeven more preferably at least 60 g/10 min.

If the layers 1 and 2 are made of an air-laid short cellulose fibrematerial, it has been found particularly preferable to use athermoplastic, polymeric, organic material composed of particles ofhigh-density polyethylene with maximum dimensions of about 400 microns,characterised by a melt flow index of about 50 g/10 min., of which thequantity distributed is between 5 g/m² and 15 g/m².

Surprisingly, it has been found that the apparent contradiction due tothe incompatibility of polyethylene, which is typically hydrophobic, andcellulose fibres is in fact translated into an advantage; in fact, itmay be assumed that, during the heating of the structure, the meltedpolyethylene particles can rapidly incorporate the cellulose fibresbelonging to the two adjacent layers by virtue of the desiredcharacteristics expressed in terms of the melt flow index, but does notinterpenetrate the fibrous mass further, and thus creates discrete andwell-defined bond points between the layers, any effect of thepolyethylene on the characteristics of the structure in terms, forexample, of its absorption capacity, being limited to a maximum extent.

The thin, layered, absorbent structure of the present invention may alsobe formed from two different fibrous layers or may comprise more thantwo fibrous layers, and consequently more than one intermediate layerformed by the mixture of particles of hydrogelling, absorbent materialand particles of thermoplastic, polymeric, organic material.

In any case, by virtue also of the continuous lines of adhesive disposedbetween the fibrous layers on the respective longitudinal edge portionswhich prevent the particulate material forming the intermediate layerfrom escaping from the longitudinal edges of the structure, thestructure has the further advantage than it can be produced separatelyand stored as it is, for example, as a continuous strip rolled in theform of a roll 1, shown in FIG. 3, which can subsequently be used on theproduction line for disposable absorbent articles, for example, sanitarytowels, which incorporate the structure as an absorbent element.

In an alternative configuration, shown in FIG. 4, the thin, layered,absorbent structure of the present invention comprises longitudinal edgeportions 8' which are particularly wide and extend laterally beyond thecontinuous lines of adhesive 10; the thin, layered, absorbent structurecan thus be incorporated in a shaped absorbent article and can be shapeditself, for example, along cutting lines indicated 12, without involvingthe need to discard the material constituting the intermediate layeralong with the scraps 13 formed from the superposed fibrous layers.

FIG. 5 is a simplified diagram of a method of producing a thin, layered,absorbent structure according to the present invention.

The reels 14, 15 supply the first and second fibrous layers 1 and 2,which may be of the same material or of different materials, in the formof respective continuous strips; the mixer/dispenser 16, which issupplied by the container 17 of hydrogelling, absorbent material and bythe container 18 of thermoplastic, polymeric, organic material, formsthe intermediate layer 5 on the surface of the first fibrous layer 1;the two extruders 19, only one of which is visible in FIG. 5, then formthe two continuous lines 10 of adhesive, for example, of the hot meltingtype, at the two sides of the intermediate layer 5 and on thelongitudinal edge portions of the first fibrous layer 1.

The first fibrous layer 1 and the intermediate layer 5 are heated tomelt the particles of polymeric, organic material 7, for example, bymeans of a radiant heating element 20; the second fibrous layer 2 isthen superposed on the first fibrous layer 1 and on the intermediatelayer 5 and the three layers, combined to form the structure, are bondedby subjecting the structure to moderate pressure by passing it betweenthe two rollers 21 and 22 bringing about the adhesion of the two fibrouslayers 1 and 2 by means of the melted particles of polymeric, organicmaterial 7 and the two continuous lines of adhesive 10. The outersurface of at least one of the two rollers 21 and 22 is preferably maderesilient by being covered with a layer, for example, of siliconerubber.

Finally, the thin, layered, absorbent structure is collected on the reel23 in the form of a continuous strip.

A disposable sanitary towel 24, shown in FIG. 6, which uses a thin,layered, absorbent structure formed according to the present invention,will be described by way of non-limiting example.

The towel 24 is of the shaped type and is constituted by an upperliquid-permeable layer 25 made of a perforated polyethylene film 0.350mm thick and 24.0 g/m² weight, a substrate 26 formed by a non-wovenfabric of carded rayon (67%) and polyester (33%) fibres intertwined bymeans of water jets, 0.700 mm thick and 50 g/m² weight, an absorbentelement 27 formed by a thin, layered, absorbent structure according tothe present invention, and a lower layer 28 which is impermeable toliquids, formed by a polyethylene film 0.025 mm thick and 24.0 g/m²weight.

The absorbent element 27, which is substantially rectangular withrounded ends, is 220 mm long and 65 mm wide with an overall thickness of1.030 mm and a weight of 176.0 g/m².

The thin, layered, absorbent structure which constitutes the absorbentelement 27 is composed of two outer dry-formed ("air laid") shortcellulose fibre layers, each with a weight of 55 g/m² and anintermediate layer of particles of a hydrogelling, absorbent materialconstituted by a polyacrylate in granules with average dimensions ofbetween 300 microns and 500 microns, of which the quantity distributedis about 50 g/m², and particles of a thermoplastic polymeric, organicmaterial constituted by high-density polyethylene in particles withmaximum dimensions of about 400 microns, having an M.F.I. of about 50g/10 min, of which the quantity distributed is about 10 g/m². The twocontinuous lines of adhesive are constituted by two lines of hot meltingadhesive about 2 mm wide.

The thin, layered, absorbent structure incorporated in the sanitarytowel performed well in restricted use tests both from the point of viewof its capacity to absorb and retain liquid and from the point of viewof the stability of the elements which make it up.

Naturally, the principle of the invention remaining the same, thedetails of construction may be varied widely from those described andillustrated without thereby departing from the scope of the presentinvention. In particular, there may be more than two layers of fibrousmaterial thus forming several pairs of fibrous layers each enclosing anintermediate layer comprising a mixture of hydrogelling, absorbentmaterial and thermoplastic material for joining the fibrous layerstogether in the manner described above.

What is claimed is:
 1. A layered, absorbent structure, characterized inthat it comprises, in combination, first and second layers (1, 2) offibrous material and an intermediate layer (5) comprising ahydrogelling, absorbent material (3) distributed between the first andsecond fibrous layers (1, 2), at least one of the first and secondlayers (1, 2) being permeable to liquids, and the intermediate layer (5)also comprising a thermoplastic material (7) which bonds at least thefirst and second fibrous layers (1, 2) together, with the intermediatelayer (5) between them, forming discrete, spaced-apart bond points (9).2. An absorbent structure according to claim 1, characterized in thatthe first and second layers (1, 2) of fibrous material extend beyond theintermediate layer (5) laterally forming longitudinal edge portions (8),and in that the first and second layers (1, 2) of fibrous material arebonded together along each of the edge portions (8) by means of apreferably continuous line of adhesive (10).
 3. An absorbent structureaccording to claim 1, characterized in that the first and second layers(1, 2) of fibrous material have a basic weight of between about 20 g/m²and about 150 g/m².
 4. An absorbent structure according to claim 1,characterized in that the quantity of the hydrogelling, absorbentmaterial (6) distributed between the first and second fibrous layers (1,2) is between about 30 g/m² and about 150 g/m².
 5. An absorbentstructure according to claim 1, characterized in that the thermoplasticmaterial (7) is a polymeric, organic material.
 6. An absorbent structureaccording to claim 1, characterized in that the quantity ofthermoplastic material (7) present is between about 5 g/m² and about 30g/m².
 7. An absorbent structure according to claim 1, characterized inthat the thermoplastic material (7) is present in the form of finelydivided material.
 8. An absorbent structure according to claim 1,characterized in that the thermoplastic material (7) has a melt flowindex (M.F.I.) of at least 25 g/10 min.
 9. A layered, absorbentstructure, characterized in that it comprises, in combination, first andsecond layers (1, 2) of a fibrous material with a basic weight ofbetween about 20 g/m² and about 150 g/m², as well as an intermediatelayer (5) comprising particles of a hydrogelling, absorbent material ofwhich a quantity of between about 30 g/m² and about 150 g/m² isdistributed between the first and second fibrous layers (1, 2), at leastone of the first and second fibrous layers (1, 2) being permeable toliquids, the first and second fibrous layers (1, 2) extending beyond theintermediate layer (5) laterally to form longitudinal edge portions (8),the intermediate layer (5) comprising a mixture of hydrogelling,absorbent material (6) and a quantity of between about 5 g/m² and about30 g/m² of a finely divided thermoplastic, polymeric, organic material(7), which has a melt flow index (M.F.I.) of at least 25 g/10 min. andis intended to fond at least the first and second fibrous layers (1, 2)together with the intermediate layer (5) between them, forming discrete,spaced-apart bond spots (9), the layered, absorbent structure also beingcharacterized in that the layers of fibrous material (1, 2) are bondedtogether along each longitudinal edge portion (8) by means of acontinuous line of adhesive (10).
 10. An absorbent structure accordingto claim 1, characterized in that it is generally rectangular.
 11. Anabsorbent structure according to claim 1, characterized in that itcomprises more than two layers of fibrous material with an intermediatelayer (5) between each pair of fibrous layers, the intermediate layercomprising a mixture of hydrogelling absorbent material (6) and finelydivided, thermoplastic material (7).
 12. An absorbent structureaccording to claim 1, characterized in that the layers of fibrousmaterial (1, 2) comprise dry-formed layers of short cellulose fibres.13. An absorbent structure according to claim 1, characterized in thatthe layers of fibrous material (1, 2) are made of different materials.14. An absorbent structure according to claim 1, characterized in thatthe thermoplastic material (7) has a melt flow index of at least 40 g/10min., preferably at least 60 g/10 min.
 15. An absorbent structureaccording to claim 1, characterized in that the thermoplastic material(7) is in the form of solid hot melting particles.
 16. An absorbentstructure according to claim 1, characterised in that the thermoplasticmaterial is polyethylene-based.
 17. An absorbent structure according toclaim 1, characterised in that the thermoplastic material is in the formof a quantity of between about 5 g/m² and 15 g/m² of polyethyleneparticles with maximum dimensions of 400 microns.
 18. An absorbentstructure according to claim 9, characterised in that the adhesion lines(10) comprise a hot melting adhesive.
 19. An absorbent structureaccording to claim 1, characterised in that its thickness is no morethan 3 mm.
 20. A disposable, absorbent article comprising an absorbentstructure according to claim 9, disposed between an upperliquid-permeable layer (15) and a lower layer (28) which is impermeableto liquids.
 21. A disposable, absorbent article according to claim 20 inthe form of a sanitary towel with a thickness no greater than about 4mm.
 22. A method of producing an absorbent structure according to claim1, characterized in that it comprises the following steps:providing thefirst layer (1) of fibrous material, providing the second layer (2) offibrous material of a width such that it can contact the first layer (1)along respective longitudinal edge portions (8), distributing a mixtureof particles of hydrogelling, absorbent material (6) as well as thefinely divided, thermoplastic material (7) on the first layer (1) offibrous material, the mixture being distributed in a manner such thatthe longitudinal edge portions (8) of the first layer (1) are left free,applying lines of adhesive (10) to the longitudinal edge portions of thefirst layer (1), and combining the first layer (1), the intermediatelayer (5) and the second layer (2) in a layered structure by theapplication of heat and pressure to melt the thermoplastic material soas to form discrete, spaced-apart bond points (9) between the firstlayer (1) and the second layer (2) with the lines of adhesive (10)bonding the first and second layers (1, 2) together along thelongitudinal edge portions (8).
 23. A method according to claim 22,characterised in that the lines of adhesive (10) are applied in the formof continuous lines.
 24. A method of producing an absorbent sanitaryarticle, characterized in that it comprises the followingsteps:providing a first continuous layer (25) of a liquid-permeable,flexable sheet material, providing a second continuous layer (28) of aflexible sheet material which is impermeable to liquids, providing atleast one generally rectangular portion of an absorbent structureaccording to any one of claims 1 to 19, and combining the firstcontinuous layer (25), the second continuous layer (28) and theabsorbent structure in a manner such that the absorbent structure isdisposed and fixed between the first and second continuous layers (25,28).